Ask people to describe concrete and the first adjectives they use, asides from “hard”, is likely to be “grey”. In fact coloured concrete is a popular choice with many architects and designers, and it has been since the 1950’s. It continues to grow in popularity on polished concrete, stamped concrete and patterned concrete. Both interior and exterior concrete can be stained or coloured. To ensure the best results, though, it is important to understand how the different processes for colouring work and what factors can affect the success of the finished product.
Integral colours are added to concrete when it is still a wet mix. The colours are principally formed from iron oxide pigments, which are either natural or synthetic. This produces colours on a red to yellow spectrum. Black and brown shades can also be achieved. Cobalt and chromium oxide are used to add blue and green tones. Naturally produced pigments will result in warmer tones, while synthetic pigments are more vibrant due to their higher tinting strength.
To tint concrete successfully, the base colour of the concrete should be taken into account. Grey and white cements will colour differently. Grey cement will result in a darker shade of colour. Due to the variations between batches, there may be some inconsistency in the final colours achieved. For a true colour to be attained, white cement should be used. This is a more expensive option, and it is usually worth making up samples in advance of full mixes. This will give a realistic idea of how the final mix will look.
The other factor which affects the final appearance of colour is the consistency of the water to cement ratio. Adding additional water will lighten the final appearance of the colour. Rapid hydration also risks lightening the tones on the concrete. In very hot or windy conditions it is worth using evaporation control agent, rather than water, to control the hydration of the slab.
It is also vital to consider any work that is going to be done to finish the surface. Sealers and densifier will have an impact on the depth of colour, as will any aggregates which are exposed.
Unlike integral colour, concrete dye is applied after the concrete has been placed. Powder based dyes need to be applied while the concrete is still curing. Solvent and water based dyes can be applied on cured, dry concrete. Dyes are available in a wide range of colours and can be tailored to the precise requirements of a project. Concrete which has been integrally coloured can be further coloured with a dye to provide an accent colour, or to deepen the shade already present in the concrete.
Water based concrete dye is the most popular solution for colouring concrete. It can be diluted to reduce the intensity of the colour. This allows for layering of the colour, creating a more natural look. It should not be allowed to form pools as this can create unwanted marks in the concrete. Water based concrete dye is the best option for projects when a more subtle effect is desired.
Solvent based dyes are stronger and will result in much bolder colours. They can be diluted to reduce the intensity of the colour. They evaporate much more rapidly than water based dyes, so they can be less forgiving to use. The vapours produced can also pose a hazard to health and can risk igniting if reasonable precautions are not taken.
Dyes are not as resistant to ultraviolet light as other forms of colour, and they do not bond to the concrete in the same way acid stains do. There is a danger of dyes washing away, especially when used on exterior concrete. They should be sealed with a UV resistant sealer to maintain their effect.
An acid based chemical stain can be used to permeate the surface of concrete, creating rich colours. Because stains are semi-transparent they will enhance the existing concrete surface. This can be a positive if the surface is of a high quality already, but it means the stain will not disguise any cracks or marks in the concrete. Colours are slightly more limited to earth tones, terra cottas and pale blue-green tones. They provide excellent UV stability, so they can be used in both interior and exterior locations. Because the stains permeate into the concrete there is no danger of the colour wearing or washing away. Suitable for use on both new and existing concrete.